Radio communication device and radio communication method

ABSTRACT

A radio communication device includes a reception unit that receives a part of data transmitted from a first communication device, via a first path including a radio channel between the radio communication device and the first communication device and that receives another part of data transmitted from the first communication device, via a second path that goes through a second communication device; a control unit that performs communication control in accordance with a state of data communication performed via the second path; and a transmission unit that transmits, to the first communication device via the first path by using control performed by the control unit in accordance with the state of the data communication performed via the second path, reception state information that specifies data that has been received or data that has not been received by the reception unit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/269,372, filed on Sep. 19, 2016, which is a continuation ofInternational Application No. PCT/JP2014/057915, filed on Mar. 20, 2014and designating the U.S., the entire contents of which are incorporatedherein by reference.

FIELD

The embodiments discussed herein are directed to a radio communicationdevice and a radio communication method.

BACKGROUND

Conventionally, in order to increase transmission capacity in a radiocommunication system (hereinafter, referred to as “system capacity”),various devices are designed. For example, in the 3^(rd) GenerationPartnership Project Radio Access Network Long Term Evolution (3GPP LTE),the technology related to increase the system capacity by making themost use of, in addition to macro cells, small cells (micro cell) isbeing discussed. Here, the cell indicates the range that is covered by aradio base station in order for a radio terminal to transmit and receivea radio signal. Furthermore, the macro cell is a cell that accommodatesa base station and in which transmission electrical power is relativelyhigh and the radio wave reach range is relatively large. Furthermore,the small cell is a cell that accommodates a base station in whichtransmission electrical power is relatively low and the radio wave reachrange is relatively small.

In the 3GPP LTE-Advanced (LTE-A), as the configuration of a radiocommunication system, for example, the configuration in which aplurality of small cells is included in a macro cell is being studied.Furthermore, the technology in which a mobile station simultaneouslyconnects a macro cell and a small cell is being studied. In addition,the technology in which a mobile station simultaneously connects twodifferent small cells is being studied. In this way, communication inwhich a mobile station simultaneously connects two different cells andperforms operations may sometimes be called dual connectivity (DualConnectivity).

When a mobile station simultaneously connects to a macro cell and asmall cell, for example, a signal of a control plane that includestherein control information on the layer 3, such as setting of atransmission path, control of handover, or the like, is transmitted andreceived to and from a base station in a macrocell (hereinafter,referred to as a “macro base station”). Furthermore, for example, asignal of a data plane that includes therein user data is transmittedand received to and from the macro base station and the base station ina small cell (hereinafter, referred to as a “small base station”). Here,the control plane may sometimes be called a control plane (ControlPlane: C plane), a Signaling Radio Bearer (SRB), or the like.Furthermore, the data plane may sometimes be called a user plane (UserPlane: U plane) or a DRB (Data Radio Bearer), or the like.

In contrast, when a mobile station simultaneously connects to twodifferent small cells, for example, the signal of the control plane istransmitted and received to and from one of the small base stations andthe signal of the data plane is transmitted and received to and from theother one of the small base stations. The signal of the data plane mayalso be transmitted and received to and from both the small basestations.

In such dual connectivity described above, a base station to which thecontrol plane is connected may sometimes be called a primary basestation. Furthermore, a base station that performs communication incooperation with the primary base station and to which the data plane isconnected may sometimes be called a secondary base station. Furthermore,these base stations may sometimes be called anchor radio base stations,assisting radio base stations, master radio base stations, or slaveradio base stations. Furthermore, in the latest trend of LTE-A, thesebase stations are respectively called a master base station and asecondary base station. In the present application, the respective basestations may sometimes be called a first communication device and asecond communication device.

Regarding sharing of functions with respect to a primary base stationand a secondary base station in dual connectivity, variousconfigurations are proposed in accordance with which of the layer isused to split a signal of the data plane. For example, there is theconfiguration in which a signal of the data plane is split in theprevious stage of the Packet Data Convergence Protocol (PDCP) layer.Furthermore, for example, there is the configuration in which a signalof the data plane is split between the PDCP layer and the Radio LinkControl (RLC) layer. Furthermore, for example, there is theconfiguration in which a signal of the data plane is split between theRLC layer and the Medium Access Control (MAC) layer. The configurationis not limited to these and it is also possible to use the configurationin which a signal of the data plane is split in each of the layers.Furthermore, for example, it is also possible to use the configurationin which some function performed in the PDCP layer is allocated to theprimary base station and the rest of the function performed in the PDCPlayer is allocated to the secondary base station. The same applied tothe function of the RLC layer and the MAC layer. Furthermore, in thelatest trend of LTE-A, it is supposed to use the configuration in whicha signal of the data plane is split between the PDCP layer and the RLClayer (Architecture 3C) and the configuration in which each of themaster base station and the small base station has the PDCP layer, theRLC layer, and the MAC layer (Architecture 1C).

The primary base station and the secondary base station that share thefunction are connected with each other by a wired or radio link. Then,the signal of the data plane that is split in the primary base stationis transmitted to the secondary base station via this link.

Non Patent Literature 1: 3GPP TS36.300 V12.0.0 (2013-12), 3rd GenerationPartnership Project.; Technical Specification Group Radio AccessNetwork; Evolved Universal Terrestrial Radio Access (E-UTRA) and EvolvedUniversal Terrestrial Radio Access Network (E-UTRAN); Overalldescription; Stage 2

Non Patent Literature 2: 3GPP TS36.211 V12.0.0 (2013-12), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channelsand modulation

Non Patent Literature 3: 3GPP TS36.212 V12.0.0 (2013-12), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing andchannel coding

Non Patent Literature 4: 3GPP TS36.213 V12.0.0 (2013-12), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layerprocedures

Non Patent Literature 5: 3GPP TS36.321 V12.0.0 (2013-12), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Medium AccessControl (MAC) protocol specification

Non Patent Literature 6: 3GPP TS36.322 V11.0.0 (2012-09), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Link Control(RLC) protocol specification

Non Patent Literature 7: 3GPP TS36.323 V11.2.0 (2013-03), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Packet DataConvergence Protocol (PDCP) specification

Non Patent Literature 8: 3GPP TS36.331 V12.0.0 (2013-12), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Radio ResourceControl (RRC); Protocol specification

Non Patent Literature 9: 3GPP TS36.413 V12.0.0 (2013-12), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1Application Protocol (S1AP)

Non Patent Literature 10: 3GPP TS36.423 V12.0.0 (2013-12), 3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network; Evolved Universal Terrestrial Radio Access Network(E-UTRAN); X2 Application Protocol (X2AP)

Non Patent Literature 11: 3GPP TR36.842 V12.0.0 (2013-12), 3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network; Study on Small Cell enhancements for E-UTRA and E-UTRAN;Higher layer aspects

However, when the dual connectivity is being performed, for example, ifa communication state is changed in communication performed between thesecondary base station and the mobile station, it is conceivable that acase of releasing the dual connectivity and switching to the singleconnectivity may occur. Namely, when the mobile station simultaneouslyconnects the macro base station and the small base station, if thecommunication state between the mobile station and the small basestation is changed, it is conceivable that the dual connectivity isreleased and the mobile station performs communication with only themacro base station. An example of a change of the communication stateincludes, for example, a failure related to data communication, theoccurrence of an error, the degradation of the radio quality, or thelike.

However, when the dual connectivity is being performed, because a signalof the data plane is transmitted from both the macro base station andthe small base station, if a case of switching to the singleconnectivity occurs, duplication or a loss of user data may possiblyoccur in the mobile station. Namely, if a case of switching from thedual connectivity to the single connectivity occurs, the macro basestation may sometimes transmit, in a duplicated manner, user data thathas already been transmitted from the small base station or maysometimes make the user data loss without transmitting the user datathat has not yet been transmitted from the small base station.

Furthermore, this problem occurs not only in a downlink, which is fromthe base station toward the mobile station, but also in an uplink, whichis from the mobile station toward the base station. Namely, when thedual connectivity is being performed, the mobile station transmits theuser data to both the macro base station and the small base station;however, after having switched to the single connectivity, the mobilestation transmits the user data only to the macro base station. At thistime, because the mobile station does not identify whether the user datathat has already been transmitted to the small base station istransferred to the macro base station, it is difficult to transmit theuser data with the proper quantity to the macro base station afterhaving switched to the single connectivity. Similarly, in also a case inwhich the mobile station performs three or more multiple connectivity,inconsistency of user data that is transmitted and received after havingswitched to the single connectivity may sometimes occur.

SUMMARY

According to an aspect of an embodiment, a radio communication deviceincludes a reception unit that receives a part of data transmitted froma first communication device, via a first path including a radio channelbetween the radio communication device and the first communicationdevice and that receives another part of data transmitted from the firstcommunication device, via a second path that goes through a secondcommunication device; a control unit that performs communication controlin accordance with a state of data communication performed via thesecond path; and a transmission unit that transmits, to the firstcommunication device via the first path by using control performed bythe control unit in accordance with the state of the data communicationperformed via the second path, reception state information thatspecifies data that has been received or data that has not been receivedby the reception unit.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a configuration example of aradio communication system according to a first embodiment;

FIG. 2 is a block diagram illustrating the configuration of the radiocommunication system according to the first embodiment;

FIG. 3 is a block diagram illustrating the layer configuration of aradio communication system according to a second embodiment;

FIG. 4 is a schematic diagram illustrating a specific example of theformat of a PDCP SR;

FIG. 5 is a schematic diagram illustrating the list of the PDU type;

FIG. 6 is a sequence diagram illustrating a connection switching methodaccording to the second embodiment;

FIG. 7 is a flowchart illustrating a process performed by a mobilestation according to the second embodiment;

FIG. 8 is a sequence diagram illustrating a connection switching methodaccording to a third embodiment;

FIG. 9 is a block diagram illustrating the hardware configuration of abase station; and

FIG. 10 is a block diagram illustrating the hardware configuration of amobile station.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of a radio communication device and a radiocommunication method disclosed in the present invention will beexplained with reference to accompanying drawings. Furthermore, thepresent invention is not limited to the embodiments. Furthermore, itgoes without saying that each of the embodiments described below can beappropriately used in combination. In a description below, theembodiments related to downlink communication and uplink communicationare described and, because the dual connectivity is often used intwo-way direction communication in which both the downlink communicationand the uplink communication are performed, it is obvious that each ofthe embodiments may also be performed in combination.

First Embodiment

FIG. 1 is a schematic diagram illustrating a configuration example of aradio communication system according to a first embodiment. The radiocommunication system illustrated in FIG. 1 includes a macro base station100, small base stations 200, and a mobile station 300.

The mobile station 300 connects to the macro base station 100 as aprimary base station. Accordingly, the mobile station 300 is connectedto the macro base station 100 by the control plane, which is representedby the arrow indicated by the solid line, and by the data plane, whichis represented by the arrows indicated by the broken lines, illustratedin FIG. 1. Furthermore, themobilestation300connects to the small basestation 200 that functions as a secondary base station. Accordingly, themobile station 300 is connected to the small base station 200 by thedata plane.

[Configuration of the Radio Communication System]

FIG. 2 is a block diagram illustrating the configuration of the radiocommunication system according to the first embodiment. As illustratedin FIG. 2, the macro base station 100 is connected to a higher layercommunication device 4 and the macro base station 100 and the small basestation 200 are connected by using, for example, an X2 interface, in awired manner. Then, the macro base station 100 and the small basestation 200 perform radio communication with the mobile station 300.

The macro base station 100 includes a communication unit 11 and acontrol unit 14. The communication unit 11 performs communication withthe small base station 200, the mobile station 300, and the higher layercommunication device 4. Namely, the communication unit 11 performs wiredcommunication with the small base station 200 and the higher layercommunication device 4 and performs radio communication with the mobilestation 300.

Specifically, the communication unit 11 includes a reception unit 12 anda transmission unit. 13. The reception unit 12 receives control data anduser data from the higher layer communication device 4. Then, thereception unit 12 outputs the received control data and the user data tothe transmission unit 13. Furthermore, the control data may also be datathat is created by the macro base station 100 by itself.

The transmission unit 13 transmits, to the mobile station 300 as radiotransmission, the control data that is addressed to the mobile station300. Furthermore, in accordance with an instruction from the controlunit 14, the transmission unit 13 transmits, to the mobile station 300as radio transmission, a part of user data addressed to the mobilestation 300 and transmits the rest of the user data to the small basestation 200.

The control unit 14 performs the overall control of the operation of thecommunication unit 11 including the reception unit 12 and thetransmission unit 13. Furthermore, the control unit 14 controls datacommunication in accordance with a communication state. When switchingfrom the dual connectivity to the single connectivity, the control unit14 refers to reception state information on the mobile station 300received from the mobile station 300 and secures consistency of datawith the mobile station 300.

Furthermore, as an example of switching to the single connectivity, forexample, the following cases are used. First, an example includes a caseof switching a communication partner from the current small base station200 to another small base station (RRC reconfiguration including theSeNB change). Furthermore, an example includes a case in which, althoughthe small base station 200 is still being set, data is transmitted toonly the macro base station 100 and is not transmitted to the small basestation 200 (RRC reconfiguration). Furthermore, an example includes acase in which the setting of the small base station 200 is deleted (RRCreconfiguration including the SeNB removal).

In the description above, regarding the timing at which the macro basestation 100 receives the reception state information from the mobilestation 300, for example, the following examples are present.

First Example

When the macro base station 100 switches from the dual connectivity tothe single connectivity, a change in the setting is transmitted, as anotification, to the mobile station 300 by using the Radio ResourceControl (RRC) signaling. Furthermore, similarly, a change in the settingis also transmitted, as a notification, to the small base station 200.It is possible to include, in the RRC signaling, an instructionindicated by the report on the reception state information. When themobile station 300 receives the RRC signaling that includes therein theinstruction indicated by the report on the reception state information,the mobile station 300 notifies the macro base station 100 of thereception state information.

Second Example

When the mobile station 300 detects a change in a communication state,the mobile station 300 transmits, at the time of detection as thetrigger, the reception state information to the macro base station 100.The report on the reception state information may also previously be setin the RRC signaling. Here, the change in the communication statecorresponds to, for example, the following cases. First, a case in whicha data loss (a packet loss in the X2 interface, data disposal due to anoverflow caused by traffic congestion in a small base station,transmission failure in a radio link, or the like) occurs in atransmission path (an X2 interface and a radio link) that is via thesmall base station 200 and thus desired data is not received in themobile station 300 (detection is possible when a timer in the PDCP layerhas expired) corresponds to the change in the communication state.Second, a case in which, retransmission of desired data transmission hasfailed in the RLC layer of the small base station 200 and degradation ofthe quality of the radio link (Radio Link Failure) is detected(detection is possible by a counter, in the RLC layer, in which themaximum number of retransmissions exceeds) corresponds to the change inthe communication state. In this case, in addition to downlinktransmission, transmission of an acknowledgment response that isassociated with the downlink transmission (the RLC STATUS REPORT that isused to transmit ACK and NACK or an acknowledgment response of TCP ifdownlink communication is Transmission Control Protocol (TCP)communication) is likely to fail. Thus, if uplink communication hasfailed, a Radio link Failure is detected in the RLC layer of the mobilestation 300. To summarize the above, the occurrence of a failure ofcommunication in the transmission path via the small base station 200 atthe time of dual connectivity is an example of a change in thecommunication state.

To summarize the above examples, there are three triggers at which themobile station 300 transmits the reception state information. Namely,the first trigger is detection of a change in the downlink communicationstate in the mobile station 300 (detection of a state change in thewired link and the radio link is possible); the second trigger isdetection of a change in the downlink communication state in the smallbase station 200 (detection of a state change in the radio link ispossible); and the third trigger is detection of a change in the uplinkcommunication state in the mobile station 300 (detection of a statechange in the radio link is possible).

Third Example

The third example is an example in combination described above.Specifically, in the second example, at least one of the three changesin the communication states described in the second example is sent tothe macro base station 100 as a notification and, if the macro basestation receives the subject notification, a change in the setting issent to the mobile station 300 as a notification by using the RRCsignaling. At this point, the macro base station 100 can include, in theRRC signaling, the instruction indicated by the report on the receptionstate information. In this way, the macro base station 100 collectivelycontrols the transmission timing of the reception state information,whereby it is possible for the mobile station 300 to avoid fromsimultaneously reporting the reception state information due to thetrigger of the “change in the downlink communication state” and thetrigger of the “change in the uplink communication state” describedabove.

In order to secure the consistency of data the control unit 14 refers tothe reception state information and performs control of transmissionthat is performed after having switched to the single connectivity suchthat the user data that has not yet been received by the mobile station300 is to be transmitted and the user data that has already beenreceived by the mobile station 300 is not to be transmitted.

Here, as a method of transmitting the user data that has not yet beenreceived by the mobile station 300, the control unit 14 may alsotransmit the data stored in the macro base station 100 withoutprocessing anything or may also transmit the data that is sent back fromthe small base station 200 and that, has not been transmitted to themobile station 300 (the data that has not been transmitted by a packetscheduler and the data that has been transmitted by the packet schedulerbut the delivery confirmation has not been checked) and may alsotransmit the data that is continuously received by the macro basestation 100 (the data that, is being transmitted over the X2 interface,which may also be referred to as fresh data).

As described above, according to the embodiment, when switching from thedual connectivity to the single connectivity, the macro base stationreceives, from the mobile station, the reception state information thatspecifies an unreceived packet and a received packet. At this point, themacro base station refers to the reception state information andperforms control such that the packet that has not yet been received bythe mobile station is to be transmitted and the packet that has alreadybeen received by the mobile station is not to be transmitted.Consequently, even if a case of switching a connection occurs, it ispossible to transmit a packet from the macro base station to the mobilestation in accordance with the reception state and it is possible tosecure the consistency of data.

Furthermore, in the first embodiment described above, a description hasbeen given of the downlink communication that is from the macro basestation 100 toward the mobile station 300; however, it is also possibleto perform the same process on the uplink communication that is from themobile station 300 toward the macro base station 100. Namely, forexample, in a case in which a communication failure occurs in an uplinktransmission path via the small base station 200 at the time of dualconnectivity, the macro base station 100 may also transmit the receptionstate information to the mobile station 300. Consequently, even in acase of the occurrence of switching to the single connectivity, it ispossible to transmit a packet from the mobile station 300 to the macrobase station 100 in accordance with the reception state and thus it ispossible to secure the consistency of data.

Second Embodiment

The configuration example of a radio communication system according to asecond embodiment is the same as that in the first embodiment (FIG. 1);therefore, descriptions thereof will be omitted. The radio communicationsystem having this configuration is often used to reduce the trafficoffload or the number of handovers.

[Configuration of the Radio Communication System]

The configuration of the radio communication system according to thesecond embodiment is the same as that in the first embodiment (FIG. 2);therefore, regarding the components having the same configuration asthose in the first embodiment, descriptions thereof in detail will beomitted.

The macro base station 100 includes the communication unit 11 and thecontrol unit 14. The communication unit 11 performs communication withthe small base station 200, the mobile station 300, and the higher layercommunication device 4. Specifically, the communication unit 11 includesthe reception unit 12 and the transmission unit 13. The reception unit12 receives control data and user data from the higher layercommunication device 4. Then, the reception unit 12 outputs the receivedcontrol data and the user data to the transmission unit 13.

Furthermore, if the small base station 200 detects an error(communication failure), the reception unit 12 receives an errordetection notification from the small base station 200. Then, if theerror detection notification is received, the reception unit 12receives, from the mobile station 300 as radio reception, receptionstate information that specifies the user data that has already beenreceived by the mobile station 300 and the user data that has not yetbeen received by the mobile station 300. Then, the reception unit 12outputs the received error detection notification and the receptionstate information to the control unit 14. Furthermore, the timing atwhich the reception unit 12 receives the reception state information andthe content of the reception state information will be described indetail later.

The transmission unit 13 transmits, to the mobile station 300 as radiotransmission, the control data addressed to the mobile station 300.Furthermore, in accordance with an instruction from the control unit 14,the transmission unit. 13 transmits, to the mobile station 300 as radiotransmission, a part of user data addressed to the mobile station 300and transmits the rest of the user data to the small base station 200.

The control unit 14 performs the overall control of the operation of thecommunication unit 11 including the reception unit 12 and thetransmission unit 13. Furthermore, if the error detection notificationis output from the reception unit 12, the control unit 14 decides totemporarily release the dual connectivity and switch to the singleconnectivity. Then, when the control unit 14 switches to the singleconnectivity, the control unit 14 refers to the reception stateinformation that was output from the reception unit 12 and decides theuser data that is to be transmitted to the mobile station 300 afterswitching to the single connectivity. Specifically, the control unit 14refers to the reception state information and controls transmission thatis performed after having switched to the single connectivity such thatthe user data that has not yet been received by the mobile station 300is to be transmitted and the user data that has already been received bythe mobile station 300 is not to be transmitted.

The small base station 200 includes a communication unit 21 and acontrol unit 24. The communication unit 21 performs communication withthe macro base station 100 and the mobile station 300. Namely, thecommunication unit 21 performs communication with the macro base station100 in a wired manner and performs radio communication with the mobilestation 300.

Specifically, the communication unit 21 includes a reception unit 22 anda transmission unit 23. The reception unit 22 receives the user datafrom the macro base station 100 via the wired connection. Then, thereception unit 22 outputs the received user data to the transmissionunit 23.

The transmission unit 23 transmits, to the mobile station 300 as radiotransmission, the user data that is output from the reception unit 22and that is addressed to the mobile station 300. Furthermore, if anerror is detected in the communication with the mobile station 300, thetransmission unit 23 transmits an error detection notification to themacro base station 100. The error detected here is, for example, anerror indicating that a reception confirmation (ACK) of the user data isnot received from the mobile station 300 even after the predeterminedtime has elapsed since the user data was transmitted to the mobilestation 300, an error indicating that, the number of retransmissions ofthe user data reaches the predetermined maximum number ofretransmissions, or the like. Furthermore, in also a case in which anerror is detected in the communication with the macro base station 100,the transmission unit 23 may also transmit the error detectionnotification to the macro base station 100.

The control unit 24 performs the overall control of the operation of thecommunication unit 21 including the reception unit 22 and thetransmission unit 23.

The mobile station 300 includes a communication unit 31 and a controlunit 34. The communication unit 31 performs communication with the macrobase station 100 and the small base station 200. Namely, thecommunication unit 31 performs the dual connectivity with the macro basestation 100 and the small base station 200 and simultaneously performsradio communication with both the base stations.

Specifically, the communication unit 31 includes a reception unit 32 anda transmission unit 33. The reception unit 32 receives, from the macrobase station 100 as radio reception, the control data and the user data.At the same time, the reception unit 32 receives, from the small basestation 200 as radio reception, the user data. Namely, the receptionunit 32 directly receives, from the macro base station 100, a part ofthe user data transmitted from the macro base station 100 and receives,via the small base station 200, the rest of the part of the user datatransmitted from the macro base station 100.

Furthermore, if an error is detected in the communication with the smallbase station 200, the reception unit 32 creates the reception stateinformation that specifies the user data that has already been receivedby the mobile station 300 from the macro base station 100 and the smallbase station 200 and the user data that has not yet been received by themobile station 300 from the macro base station 100 and the small basestation 200. Then, the reception unit 32 outputs the created receptionstate information to the control unit 34.

The transmission unit 33 acquires, via the control unit 34, thereception state information created by the reception unit 32. Then, thetransmission unit 33 transmits, as radio transmission, the acquiredreception state information to the macro base station 100.

The control unit 34 performs the overall control of the operation of thecommunication unit 31 including the reception unit 32 and thetransmission unit 33. Furthermore, the control unit 34 monitors thereception unit 32 and detects an error occurring in the communicationwith the small base station 200. The error detected here is, forexample, an error indicating that desired user data is not received evenafter the predetermined time since the user data was received from thesmall base station 200, an error indicating that the number ofretransmissions of the user data reaches the predetermined maximumnumber of retransmissions, or the like. Furthermore, if the control unit34 detects an error occurring in the reception unit 32, the control unit34 acquires the reception state information created by the receptionunit 32 and then outputs the reception state information to thetransmission unit 33.

Here, an error indicating that desired user data is not received can bedetected in, for example, the PDCP layer. Specifically, if missing ofreception order of the user data is detected, i.e., if it is detectedthat unreceived user data is present (also called out-of-orderdelivery), a timer is started in order to wait an arrival of a firstunreceived packet. Then, if the subject packet is received before thetimer has expired, it is determined that the reception has beensuccessful; however, if the subject packet is not received, it isdetermined that an error occurs. After that, the same process isperformed on the subsequent unreceived packets.

Furthermore, an error indicating that the number of retransmissions ofthe user data reaches the predetermined maximum number ofretransmissions can be detected in, for example, the RLC layer.Specifically, in the RLC layer, retransmission (Automatic RepeatRequest) control is prescribed, the retransmission related to the userdata in which an error occurs in the radio transmission is performed. Ifthe retransmission is successful within a predetermined number of times,it is determined that the reception has been successful; however, if thenumber of retransmissions exceeds the predetermined number of times, itis determined that an error occurs. In this case, conventionally, it isdetermined that a Radio Link Failure (RLF) has occurred and RRCreconnection (RRC Connection Re-establishment) is performed.Furthermore, in the dual connectivity, in particular, the RLF occurs onthe small base station 200 side, the RRC reconnection is not performed;however, the communication failure (RLC failure) in the RLC layer istransmitted to the macro base station 100 as a notification.Furthermore, as described in the first embodiment, the communicationfailure in the RLC layer can be detected by the RLC on the downlinktransmission side and the RLC on the downlink reception side.

In the embodiment, if these errors are detected, the reception stateinformation is transmitted from the mobile station 300 to the macro basestation 100. Consequently, the reception state information can promptlybe transmitted to the macro base station 100 when compared with thetechnology in which the reception state information is transmitted whenan amount of retention in the buffer in, for example, the mobile station300 exceeds a predetermined threshold. Furthermore, if a large amount ofloss of user data is detected in the PDCP layer, the timer is expiredfor each lost packet, the reception state information is accordinglytransmitted, and thus the signaling overhead is increased. Therefore,for example, a prohibit timer (Prohibit Timer) may also separately beset such that reception state information is not frequently transmitted.Specifically, it is determined, by using the prohibit timer, whether acertain period of time has elapsed since the reception state informationwas transmitted last time and it may also perform control such that thereception state information is not transmitted again during the timeperiod for which the certain period of time has not elapsed.

[Process Performed in the Radio Communication System]

In the following, a description will be given of the process performed,in the radio communication system, by the macro base station 100, thesmall base station 200, and the mobile station 300. The macro basestation 100, the small base station 200, and the mobile station 300perform communication by using the link layer protocols associated witha plurality of link layers. Namely, for example, the link layerprotocols associated with the Packet Data Convergence Protocol (PDCP)layer, the Radio Link Control (RLC) layer, the Medium Access Control(MAC) layer, the Physical (PHY) layer, and the like are used. FIG. 3 isa block diagram illustrating the layer configuration of a radiocommunication system according to a second embodiment.

Here, first, a process performed by the macro base station 100 relatedto transmitting and receiving user data will be described. Asillustrated in FIG. 3, the communication unit 11 in the macro basestation 100 includes a PDCP layer 101, an RLC layer 102, an RLC layer103, and a MAC layer 104. The RLC layer 102 is an RLC layer for thedownlink and the RLC layer 103 is an RLC layer for the uplink.Furthermore, the macro base station 100 may also include a layer, whichis not illustrated, such as the PHY layer or the like.

The communication unit 11 receives user data from the higher layercommunication device 4. Then, the communication unit 11 adds, in thePDCP layer 101, sequence numbers to the packets in the received userdata. At this time, the communication unit 11 adds an odd number to thepacket that is output to, for example, the RLC layer 102 and adds aneven number to the packet that is transmitted to the small base station200. These sequence numbers are also used at the time of, for example,handover or the like. Furthermore, the communication unit 11 performs,in the PDCP layer 101, header compression, a security check, andencryption with respect to the user data.

Then, the communication unit 11 outputs, from the PDCP layer 101 to theRLC layer 102, the packets to each of which the odd number is added.Furthermore, the communication unit 11 transmits, to the small basestation 200 via the wired connection, the packets to each of which theeven number is added. Consequently, for example, the packets to whichsequence numbers #1, #3, #5, #7, and . . . are added are output to theRLC layer 102 and the packets to which the sequence numbers #2, #4, #6,#8 and . . . are added are transmitted to the small base station 200.

Furthermore, the numbers that are added to the packets are not alwaysthe sequence numbers in ascending order. Namely, other numbers or thelike may also be added to the packets as long as each of the packets canbe identified and identifiers indicate the order of all of the packetsin the user data. In the following, a description will continuously begiven with the assumption that the continuous sequence numbers inascending order are added to the packets.

The communication unit 11 acquires, in the RLC layer 102 from the PDCPlayer 101, the packets to each of which an odd number is added. Then,the communication unit 11 divides or integrates, in the RLC layer 102 asneeded, the packets and adds a header of the RLC layer, whereby thecommunication unit 11 creates the packets of the RLC layer (hereinafter,referred to as “RLC packets”).

Then, the communication unit 11 outputs the RLC packets from the RLClayer 102 to the MAC layer 104 in accordance with the scheduling that isset in the MAC layer 104. Then, the communication unit 11 assembles, inthe MAC layer 104 by using the RLC packets, data for transmission.Namely, for example, by dividing or integrating the RLC packets asneeded and by adding a header of the MAC layer, the packets of the MAClayer (hereinafter, referred to as “MAC packets”) are created. Then, thecommunication unit 11 transmits the MAC packets in accordance with thescheduling from the MAC layer 104 to the mobile station 300 via a PHYlayer or the like that is not illustrated.

In contrast, when user data is received from the mobile station 300, thecommunication unit 11 receives, in the MAC layer 104, the user data fromthe mobile station 300. Then, the communication unit 11 reconstructs(reassembles), in the MAC layer 104, the received user data and dividesor integrates, in the RLC layer 103, the received user data.Furthermore, the communication unit 11 corrects, in the RLC layer 103,the order of pieces of the data by using the header of the RLC layer andoutputs the user data from the RLC layer 103 to the PDCP layer 101.

In the following, a process performed by the macro base station 100 whenthe error detection notification is received from the small base station200 will be described.

If an error is detected in the small base station 200, the communicationunit 11 acquires, in the PDCP layer 101, the error detectionnotification transmitted from the small base station 200. Furthermore,the communication unit 11 acquires, in the PDCP layer 101, the receptionstate information that is transmitted from the mobile station 300 viathe MAC layer 104 and the RLC layer 103. The reception state informationincludes information that can specify, at the time point at which anerror is detected, the packet that has already been received by themobile station 300 and the packet that has not yet been received by themobile station 300.

If the error detection notification is received by the communicationunit 11, the control unit 14 decides to release the dual connectivitythat is set with the mobile station 300 and switch to the singleconnectivity. Then, the control unit 14 transmits a dual connectivityrelease notification indicating a release of the dual connectivity tothe small base station 200 and the mobile station 300 via thecommunication unit 11. Furthermore, the control unit 14 refers to thereception state information received by the communication unit 11 andnotifies the PDCP layer 101 of the packets that are to be transmitted tothe mobile station 300. Namely, the control unit 14 notifies the PDCPlayer 101 of the sequence numbers of the packets that have not yet beenreceived by the mobile station 300 and that are indicated by thereception state information and then notifies the PDCP layer 101 of thesequence numbers of the packets that have already been received as thesequence numbers of the packets that are not needed to be transmitted.

Then, while excluding the packets that have already been received by themobile station 300 and that are not needed to be transmitted, thecommunication unit 11 outputs, from the PDCP layer 101 to the RLC layer102, the packets that have not yet been received by the mobile station300 with sequence numbers in ascending order. Then, the communicationunit 11 creates, in the RLC layer 102, the RLC packets; creates, in theMAC layer 104, the MAC packets; and transmits the MAC packets to themobile station 300. Consequently, after having been switched from thedual connectivity to the single connectivity, the mobile station 300 canreceive the packets without duplication and loss.

In the following, a process performed by the small base station 200related to transmitting and receiving user data will be described. Asillustrated in FIG. 3, the communication unit 21 in the small basestation 200 includes an RLC layer 201, an RLC layer 202, and a MAC layer203. The RLC layer 201 is an RLC layer for the downlink and the RLClayer 202 is an RLC layer for the uplink. Furthermore, the small basestation 200 may also include a layer, such as a PHY layer or the like,that is not illustrated.

The communication unit 21 receives, in the RLC layer 201, the packetsthat are transmitted from the PDCP layer 101 in the macro base station100 via the wired connection. As described above, in the packets, theeven numbers are added. Then, the communication unit 21 divides orintegrates, in the RLC layer 201 as needed, the packets and adds theheader of the RLC layer, whereby the communication unit 21 creates theRLC packets.

Thereafter, the communication unit 21 outputs the RLC packets from theRLC layer 201 to the MAC layer 203 in accordance with the schedulingthat is set in the MAC layer 203. Then, the communication unit 21assembles, in the MAC layer 203, data for transmission by using the RLCpackets. Namely, for example, by dividing or integrating the RLC packetsas needed and by adding the header of the MAC layer, the MAC packets arecreated. Then, the communication unit 21 transmits the MAC packets inaccordance with the scheduling fromtheMAClayer203 to the mobile station300 via the PHY layer or the like that is not illustrated.

In contrast, when user data is received from the mobile station 300, thecommunication unit 21 receives, in the MAC layer 203, the user data fromthe mobile station 300. Then, the communication unit 21 reconstructs(reassembles), in the MAC layer 203, the received user data and dividesor integrates, in the RLC layer 202, the received user data.Furthermore, the communication unit 21 corrects, in the RLC layer 202,the order of the pieces of the data by using the header of the RLC layerand transmits the user data from the RLC layer 202 to the macro basestation 100.

In the following, a process performed by the small base station 200 whenan error is detected will be described.

If the MAC packets are transmitted from the MAC layer 203 to the mobilestation 300, the control unit 24 starts the timer that measures thepredetermined time and waits the reception confirmation (ACK) receivedfrom the mobile station 300. At this time, if the MAC packets are notcorrectly received by the mobile station 300, the reception confirmation(ACK) is not received by the time at which the timer is expired.Consequently, if the reception confirmation (ACK) is not received by thetime at which the timer is expired, the control unit 24 detects that anerror has occurred.

Furthermore, the control unit 24 monitors the number of retransmissionsof data between the mobile station 300 and, if the number ofretransmissions reaches the predetermined maximum number ofretransmissions, the control unit 24 may also detects that an error hasoccurred. Furthermore, if a desired packet is not received after thepredetermined time has elapsed since a packet was received from themacro base station 100 via the wired connection, the control unit 24 mayalso detects an error has occurred. If the control unit 24 detects anerror, the communication unit 21 transmits an error detectionnotification to the macro base station 100 via the wired connection.

Thereafter, if it is decided by the macro base station 100 that the dualconnectivity is released, the communication unit 21 receives a dualconnectivity release notification from the macro base station 100. Afterthe dual connectivity release notification is received, because theradio communication between the small base station 200 and the mobilestation 300 is not performed, the communication unit 21 stopstransmitting and receiving the data.

In the following, a process performed by the mobile station 300 relatedto transmitting and receiving the user data will be described. Asillustrated in FIG. 3, the communication unit 31 in the mobile station300 includes MAC layers 301 and 302, RLC layers 303 to 306, and a PDCPlayer 307. Because the mobile station 300 has a function ofsimultaneously receiving user data from two base stations, the MAClayers and the RLC layers are provided in association with each of thebase stations. Namely, the MAC layer 301,the RLC layer 303, the RLClayer 304, and the PDCP layer 307areused to transmit and receive userdata to and from the macro base station 100. Furthermore, the MAC layer302, the RLC layer 305, the RLC layer 306, and the PDCP layer 307 areused to transmit and receive user data to and from the small basestation 200. The RLC layers 303 and 305 are RLC layers for the downlinkand the RLC layers 304 and 306 are RLC layers for the uplink.Furthermore, the mobile station 300 may also include a layer, which isnot illustrated, such as the PHY layer or the like.

The communication unit 31 receives, in the MAC layer 301, user data (MACpacket) from the macro base station 100. Then, the communication unit 31reconstructs (reassembles), in the MAC layer 301, the received user dataand divides and integrates, in the RLC layer 303, the received userdata. Furthermore, the communication unit 31 corrects, in the RLC layer303, the order of pieces of data by using the header of the RLC layerand outputs the user data from the RLC layer 303 to the PDCP layer 307.Then, the communication unit 31 releases, in the PDCP layer 307, thedecoding, the security check, and the header compression with respect tothe user data.

Similarly, the communication unit 31 receives, in the MAC layer 302, theuser data (MAC packet) from the small base station 200. Then, thecommunication unit 31 reconstructs (reassembles), in the MAC layer 302,the received user data and divides and integrates, in the RLC layer 305,the received user data. Furthermore, the communication unit 31 corrects,in the RLC layer 305, the order of pieces of data by using the header ofthe RLC layer and outputs the user data from the RLC layer 305 to thePDCP layer 307. Then, the communication unit 31 releases, in the PDCPlayer 307, the decoding, the security check, and the header compressionperformed on the user data.

In contrast, when the user data is transmitted to the macro base station100 and the small base station 200, the communication unit 31 adds, inthe PDCP layer 307, the sequence numbers to the packets in the userdata. Furthermore, the communication unit 31 performs, in the PDCP layer307, the header compression, the security check, and the encryption onthe user data.

Then, the communication unit 31 outputs some packets from the PDCP layer307 to the RLC layer 304 and outputs the rest of packets from the PDCPlayer 307 to the RLC layer 306. Then, in the RLC layers 304 and 306, bydividing or integrating the packets as needed and by adding the headerof the RLC layer, the communication unit 31 creates the packets of theRLC layer (hereinafter, referred to as “RLC packets”).

Then, the communication unit 31 outputs the RLC packets from the RLClayers 304 and 306 to the MAC layers 301 and 302 in accordance with thescheduling that is set in the MAC layers 301 and 302. Then, thecommunication unit 31 assembles, in the MAC layers 301 and 302, thepieces of data for transmission by using the RLC packets. Namely, forexample, by dividing or integrating the RLC packets as needed and byadding the header of the MAC layers, the MAC packets are created. Then,the communication unit 31 transmits, in accordance with the scheduling,the MAC packets from the MAC layers 301 and 302 to the macro basestation 100 and the small base station 200 via the PHY layer or the likethat is not illustrated.

In the following, a process performed by the mobile station 300 when anerror is detected will be described.

The control unit 34 monitors reception of user data in the communicationunit 31 and, if, for example, even if the predetermined time has elapsedafter the packets to each of which a sequence number is added werereceived, a packet that has not yet been received and that has asequence number that is previous to that added to the already receivedpackets is present in the PDCP layer 307, the control unit 34 detectsthat an error has occurred. Furthermore, the control unit 34 monitorsthe number of retransmissions of data between the macro base station 100and each of the small base stations 200 and, if the number ofretransmissions reaches the predetermined maximum number ofretransmissions, the control unit. 34 may also detect that an error hasoccurred. Then, if an error is detected at the reception of datatransmitted from the small base station 200, the control unit 34instructs the communication unit 31 to create reception stateinformation.

The communication unit 31 that has received the instruction acquires, inthe PDCP layer 307, the sequence numbers that are added to the alreadyreceived packets. Namely, because sequence numbers added in the PDCPlayer 101 of the macro base station 100 are added to each of thepackets, the sequence number added by the macro base station 100 isacquired in the PDCP layer 307. Then, the communication unit 31 confirmsin the PDCP layer 307 from the acquired sequence number, the sequencenumbers of the packets that have not yet been received.

Then, the communication unit 31 creates, in the PDCP layer 307,reception state information that indicates the smallest sequence numberfrom among the sequence numbers of the packets that have not yet beenreceived and that indicates whether the predetermined number of packetsto which the sequence numbers subsequent to the smallest sequence numberare added have been received. Specifically, for example, considering thecase in which the packets with the sequence numbers #1, #3, #5, and #7are transmitted from the macro base station 100 and the packets with thesequence numbers #2, #4, #6 and #8 are transmitted from the small basestation 200. Here, for example, it is assumed that the packet with thesequence number #2 transmitted from the small base station 200 is notreceived by the mobile station 300 and an error is detected. In thiscase, the packets with the sequence numbers #1, #3, #5, and #7transmitted from the macro base station 100 are received by the mobilestation 300 and the packets with the sequence numbers #2, #4, #6, and #8are not received by the mobile station 300.

Thus, in the PDCP layer 307, the reception state information thatindicates the smallest sequence number #2 from among the sequencenumbers added to the packets that have not yet been received and thatindicates whether the packets with the sequence numbers #3 to #8 havebeen received. Accordingly, here, the reception state information thatindicates that the packets with the sequence numbers #3, #5, and #7 havealready been received and that indicates that the packets with thesequence numbers #4, #6, and #8 have not yet been received is created.The reception state information indicates that the sequence numbers #2,#4, #6, and #8 have not yet been received by the mobile station 300 andneed to be transmitted again from the macro base station 100 afterswitching to the single connectivity.

Furthermore, the above description has been given with the assumptionthat the continuous sequence numbers in ascending order are added to thepackets in the PDCP layer 101. However, as described above, in the PDCPlayer 101, the sequence numbers are not always needed to be added to thepackets. If other identifiers are added to the packets in the PDCP layer101, the reception state information includes the identifier of thepacket (a first unreceived packet) closest to the top from amongunreceived packets in the entire user data. Furthermore, the receptionstate information in this case includes information indicating whetherthe predetermined number of packets that are subsequent to the packetthat is closest to the top in the entire user data have been received.

When the reception state information is created in the PDCP layer 307,the communication unit 31 transmits the reception state information tothe macro base station 100. Consequently, in the macro base station 100,including the packets that go through the small base station 200, it ispossible to be aware of the reception state of the packets in the mobilestation 300. Consequently, the macro base station 100 can transmit thepackets with the proper quantity to the mobile station 300 even afterreleasing the dual connectivity and switching to the single connectivityand can prevent duplication and loss of user data.

[Specific Example of the Reception State Information]

As described above, if an error is detected in the communication betweenthe small base station 200 and the mobile station 300, the receptionstate information is transmitted from the mobile station 300 to themacro base station 100. For this reception state information, forexample, a PDCP status report (hereinafter, referred to as a “PDCP SR”)can be used.

FIG. 4 is a schematic diagram illustrating a specific example of theformat of PDCP SR. A PDCP SR 400 indicated on the upper portionillustrated in FIG. 4 is a PDCP SR for a 12-bit sequence number.Furthermore, the PDCP SR 410 indicated on the medium portion illustratedin FIG. 4 is a PDCP SR for a 15-bit sequence number. Furthermore, thePDCP SR 420 indicates on the lower portion illustrated in FIG. 4 is aPDCP SR for a 7-bit sequence number.

As indicated in the drawings, in the PDCP SR, the size of the sequencenumber is different depending on the data that is transmitted andreceived. Specifically, for example, in Voice of Internet Protocol(VoIP) or the like, a 7-bit sequence number is sometimes used.Consequently, each of the PDCP SRs 400, 410, and 420 illustrated in FIG.4 has a First Missing Sequence number (FMS) fields 401, 411, and 421with different sizes. The FMS fields 401, 411, and 421 are the fieldsthat store therein the sequence number of the packet closest to the topin the entire of the user data from among the unreceived packets.

Namely, for example, in the PDCP SR 400, the sequence number of thepacket that is closest to the top in the entire of the user data fromamong the unreceived packets is stored in the FMS field 401 by themobile station 300. In other words, in the FMS field 401, from among thepackets in the user data that have not yet been received by the smallbase station 200, the sequence number of the packet that is to bereceived earliest is stored. In the following, the packets in whichsequence numbers are stored in the FMS fields 401, 411, and 421 aresometimes referred to as the “FMS packets”.

Furthermore, the fields of Bitmap₁ to Bitmap_(N) in the PDCP SRs 400,410, and 420 store therein reception status indicating whether thepackets that are subsequent to the FMS packet have been received.Specifically, for example, regarding the 1 ^(st) to the N^(th) (N is aninteger equal to or greater than 1) packets after the FMS packets, if asubject packet is received by the mobile station 300, “1” is stored and,if the subject packet is not received by the mobile station 300, “0” isstored.

For example, as the example described above, considering the case inwhich the packets with the sequence numbers #1, #3, #5, and #7 aretransmitted from the macro base station 100 and the packets with thesequence numbers #2, #4, #6, and #8 are transmitted from the small basestation 200. Here, if the packet with the sequence number #2 is notreceived by the mobile station 300 and an error is detected, the packetwith the subsequent sequence numbers #4, #6, and #8 are not alsoreceived from the small base station 200. Consequently, in the mobilestation 300, the packets with the sequence numbers #1, #3, #5, and #7have already been received and the packets with the sequence numbers #2,#4, #6, and #8 have not yet been received.

In this case, because the FMS packet is the packet with the sequencenumber #2, in the FMS fields 401, 411, and 412 in the PDCP SRs 400, 410,and 420, respectively, the sequence number #2 is stored. Furthermore,regarding the packets after the FMS packet, the packets with thesequence numbers #3, #5, and #7 have already been received and thepackets with the sequence numbers #4, #6, and #8 have not yet beenreceived. Consequently, “1”, “0”, “1”, “0”, “1”, and “0” are stored inthe respective fields of Bitmap₁ to Bitmap₆.

Furthermore, in the PDU Type field in each of the PDCP SRs 400, 410, and420 illustrated in FIG. 4, information related to the type of theProtocol Data Unit (PDU) used as the PDCP SRs 400, 410, and 420 isstored. Specifically, for example, as illustrated in FIG. 5, the bitthat represents the type of this PDU is stored in the PDU Type field.Namely, if the bits of “000” are stored in the PDU Type field, thisindicates that this PDU is the PDCP status report, such as the PDCP SRs400, 410, and 420.

In contrast, if the bits of “001” are stored in the PDU Type field, thisindicates that, this PDU is a scattering ROHC feedback packet(Interspersed ROHC feedback packet). The Interspersed ROHC feedbackpacket includes therein feedback information with respect to the PDU inthe PDCP layer transmitted from the reception side.

Furthermore, the bits of “010” to “111” that are possibly be stored inthe PDU Type field remain as reserved bits. Accordingly, in theembodiment, in the reception state information that is transmitted fromthe mobile station 300 when an error is detected, it is also possible toallocate a bit that is different from the normal PDCP SR and store thebit in the PDU Type field.

[Connection Switching Method]

In the following, a method of switching a connection from the dualconnectivity to the single connectivity according to the secondembodiment will be described with reference to the sequence diagramillustrated in FIG. 6.

When the mobile station 300 performs dual connectivity with the macrobase station 100 and the small base station 200, a part of the user datais transmitted, as radio transmission, from the communication unit 11 inthe macro base station 100 to the mobile station 300. Furthermore, therest of the user data is transmitted to the small base station 200 viathe wired connection (Step S101) and transmitted, as radio transmission,from the communication unit 21 in the small base station 200 to themobile station 300. For the packets of these pieces of the user data,the sequence numbers indicating the order of the pieces of the data areattached in the communication unit 11 in the macro base station 100 and,for example, the packets to each of which an odd number is added aredirectly transmitted, as radio transmission, to the mobile station 300from the macro base station 100 and the packets to each of which an evennumber is added are transmitted, as radio transmission, to the mobilestation 300 via the small base station 200.

Then, if the packets transmitted, as the radio transmission, from thesmall base station 200 is not received by the mobile station 300 (StepS102), an error is detected by the control unit 24 in the small basestation 200 (Step S103). This error is detected when, for example, areception confirmation (ACK) with respect to the packets transmitted, asthe radio transmission, from the small base station 200 are nottransmitted back from the mobile station 300. Furthermore, an error isalso detected by the control unit 24 in the small base station 200 in acase in which the number of retransmissions with the mobile station 300reaches the predetermined maximum number of retransmissions, a case inwhich a subsequent packet is not received even after the predeterminedtime has elapsed since a packet was received from the macro base station100, or the like.

In contrast, if a packet from the small base station 200 is notreceived, an error is also detected by the control unit 34 in the mobilestation 300 (Step S104). This error is detected in a case in which, forexample, even after the predetermined time has elapsed since the packetsto each of which a sequence number is added were received from the smallbase station 200, an unreceived packet to which a sequence number thatis previous to that added to the received packets is present.Furthermore, an error is also detected by the control unit 34 in themobile station 300 in a case in which the number of retransmissions withthe small base station 200 reaches the predetermined maximum number ofretransmissions, or the like.

If an error is detected by the control unit 24 in the small base station200, an error detection notification is transmitted to the macro basestation 100 (Step S105). Furthermore, if an error is detected by thecontrol unit 34 in the mobile station 300, reception state informationis created by the communication unit 31. The reception state informationincludes therein a sequence number of the packet that is closest to thetop in the order of the packets that have not yet been received by themobile station 300 and includes therein the reception status indicatingwhether the predetermined number of packets that are subsequent to thepacket that is closest to the top packet have been received. Then, thecreated reception state information is transmitted from thecommunication unit 31 in the mobile station 300 to the macro basestation 100 (Step S106).

In the macro base station 100, in response to the error detectionnotification transmitted form the small base station, it. is decided bythe control unit 14 that the dual connectivity is temporarily releasedand is switched to the single connectivity. Namely, it is decided thattransmission of user data via the small base station 200 is temporarilystopped and all of the pieces of the user data are directly transmittedto the mobile station 300 by the macro base station 100. Then, a dualconnectivity release notification indicating the release of the dualconnectivity is transmitted to the small base station 200 by thecommunication unit 11 (Step S107). The small base station 200 that hasreceived the dual connectivity release notification stops, after that,the transmission of the user data with respect to the mobile station300.

Furthermore, the dual connectivity release notification is alsotransmitted from the communication unit 11 in the macro base station 100to the mobile station 300 (Step S108). The mobile station 300 that hasreceived the dual connectivity release notification receives, afterthat, all of the pieces of the user data from the macro base station100.

After having switched to the single connectivity, if the macro basestation 100 transmits the user data to the mobile station 300, thereception state information received from the control unit 14 isreferred to by the mobile station 300.Then, the sequence numbers of thepackets that have not yet been received by the mobile station 300 aretransmitted, as a notification, from the control unit 14 to thecommunication unit 11 and the packets with the notified sequence numbersare sequentially transmitted from the communication unit 11 to themobile station 300.

In this way, in the embodiment, if an error is detected in atransmission path for the user data via the small base station 200, thereception state information is transmitted from the mobile station 300to the macro base station 100. Then, the macro base station 100 releasesthe dual connectivity and refers to the reception state information andtransmits the user data that has not yet been received by the mobilestation 300 to the mobile station 300 with the proper quantity.Consequently, even when switching from the dual connectivity to thesingle connectivity occurs, it is possible to prevent duplication andloss of user data that is to be received by the mobile station 300.

[Specific Example of a Process Performed by the Mobile Station at theTime of Error Detection]

In the following, a process performed by the mobile station 300 when anerror is detected, at the time of dual connectivity, in a transmissionpath via the small base station 200 will be described with reference tothe flowchart illustrated in FIG. 7.

The control unit 34 in the mobile station 300 monitors communicationbetween the small base station 200 and the mobile station 300 anddetermines whether an error has occurred (Step S201). Namely, if thepackets to each of which a sequence number is added are received fromthe small base station 200, the control unit 34 determines whether anunreceived packet with a sequence number that is previous to that, addedto the received packet is present. Then, if an unreceived packet ispresent, the control unit 34 starts a predetermined timer and determineswhether the unreceived packet is received before the predetermined timehas elapsed. As the result of the determination, if an unreceived packetis not present or the packet is correctly received before thepredetermined time has elapsed, the control unit 34 determines that noerror is detected (No at Step S201) and continuously monitors thecommunication.

Then, if an error is detected (Yes at Step S201), the control unit 34instructs the communication unit 31 to create a PDCP SP. that isreception state information. In response to this instruction, thecommunication unit 31 creates a PDCP SR that includes therein a sequencenumber of the packet that is the closest to the top from amongunreceived packets in the entire user data and that includes therein areception status indicating whether the predetermined number of packetsthat are subsequent to the packet that is the closest to the top packethave been received (Step S202). Then, the communication unit 31transmits the created PDCP SR to the macro base station 100 (Step S203).

Thereafter, the communication unit 31 receives a dual connectivityrelease notification from the macro base station 100 indicating that thedual connectivity is decided to be temporarily released (Step S204).After receiving the dual connectivity release notification, thecommunication unit 31 starts the single connectivity with the macro basestation 100 (Step S205) and receives, from the macro base station 100,all of the pieces of the user data addressed to the mobile station 300.At this time, because the macro base station 100 refers to the PDCP SRand transmits the user data that has not yet been received by the mobilestation 300 with the proper quantity, the communication unit 31 receivesthe user data from the macro base station 100 without duplication andloss.

As described above, according to the embodiment, if an error isdetected, at the time of dual connectivity, in the transmission path viathe small base station, the mobile station transmits, to the macro basestation, the reception state information that specifies an unreceivedpacket and a received packet. Then, the macro base station releases thedual connectivity, switches to the single connectivity, refers to thereception state information, and transmits the packets that have not yetbeen received by the mobile station. Consequently, even if switching ofconnection occurs, it is possible to transmit packets with the properquantity from the macro base station to the mobile station and it ispossible to prevent duplication and loss of user data.

Furthermore, in the second embodiment, it is assumed that the errordetection notification is transmitted from the small base station 200 tothe macro base station 100; however, the error detection notificationmay also be transmitted from the mobile station 300 to the macro basestation 100. In this case, the error detection notification may also betransmitted to the macro base station 100 together with the receptionstate information.

Third Embodiment

In the second embodiment, a description has been given of the connectionswitching method performed in a case in which an error is detected in adownlink from the macro base station 100 and the small base station 200toward the mobile station 300. However, in also a case in which an erroris detected in an uplink from the mobile station 300 toward the macrobase station 100 and the small base station 200, it is also possible totemporarily release the dual connectivity and switch to the singleconnectivity. Thus, in the third embodiment, a description will be givenof a connection switching method performed in a case in which an erroris detected in an uplink.

The configuration of the radio communication system according to theembodiment is the same as that described in the second embodiment;therefore, descriptions thereof will be omitted. The embodiment differsfrom the second embodiment in that an error is detected when a packettransmitted from the mobile station 300 to the smallbasestation200 isnot received by the macro base station 100.

FIG. 8 is a sequence diagram illustrating a connection switching methodaccording to a third embodiment. As illustrated in FIG. 8, when themobile station 300 performs the dual connectivity with the macro basestation 100 and the small base station 200, a part of the user data isdirectly transmitted, as radio transmission, from the communication unit31 in the mobile station 300 to the macro base station 100 (Step 5301).Furthermore, the rest, of the user data is transmitted to the macro basestation 100 via the small base station 200.

Sequence numbers are added to these packets in the user data in thecommunication unit. 31 in the mobile station 300 and, for example, thepackets to each of which an odd number is added are directlytransmitted, as radio transmission, to the macro base station 100 andthe packets to each of which an even number is added are transmitted tothe macro base station 100 via the small base station 200.

Then, if the packet transmitted via the small base station 200 is notreceived by the macro base station 100 (Step 5302), an error is detectedby the control unit 14 in the macro base station 100 (Step S303). Thiserror is detected in a case in which, for example, even after thepredetermined time has elapsed since the packets to each of which thesequence number is added were received from the mobile station 300 viathe small base station 200, an unreceived packet with a sequence numberthat is previous to the sequence numbers that are added to the receivedpackets is present. This error detection can be performed in, asdescribed above, for example, the PDCP layer. Furthermore, this errordetection can also be performed in, for example, the RLC layer, and in acase in which the error detection is performed in RLC, it is alsopossible to detect an error by the small base station 200 or the mobilestation 300. If an error is detected by the small base station 200 orthe mobile station 300, a notification indicating the occurrence of anerror is transmitted to the macro base station 100.

If an error is detected by the control unit 14 in the macro base station100, the reception state information is created by the communicationunit 11. The reception state information includes therein a sequencenumber of the packet that is closest to the top from among the packets,in the entire user data, that have not yet been received by the macrobase station 100 and includes therein the reception status indicatingwhether the predetermined number of packets subsequent to the unreceivedpacket closest to the top have been received. Then, the createdreception state information is transmitted from the communication unit11in the macrobasestation100 to the mobile station 300 (Step S304).

Furthermore, in the macro base station 100, because an error is detectedby the control unit 14, it is decided that the dual connectivity istemporarily released and is switched to the single connectivity. Namely,it is decided that reception of the user data via the small base station200 is temporarily stopped and decided that all of the pieces of theuser data are directly transmitted from the mobile station 300 to themacro base station 100. Then, the dual connectivity release notificationindicating that the dual connectivity is released is transmitted to thesmall base station 200 by the communication unit 11(Step S305).The smallbase station200 that has received the dual connectivity releasenotification stops, after that, reception of the user data transmittedfrom the mobile station 300.

Furthermore, the dual connectivity release notification is alsotransmitted from the communication unit 11 in the macro base station 100to the mobile station 300 (Step S306). The mobile station 300 that hasreceived the dual connectivity release notification directly transmits,after that, as radio transmission, all of the pieces of the user data tothe macro base station 100.

After having switched to the single connectivity, when the mobilestation 300 transmits the user data to the macro base station 100, thereception state information received form the macro base station 100 isreferred to by the control unit 34. Then, the sequence numbers of thepackets that have not been received by the macro base station 100 aretransmitted, as a notification, from the control unit 34 to thecommunication unit 31 and the packets with the notified sequence numbersare sequentially transmitted from the communication unit 31 to the macrobase station 100.

As described above, in the embodiment, if an error is detected in thetransmission path of the user data via the small base station 200, thereception state information is transmitted from the macro base station100 to the mobile station 300. Then, after the macro base station 100releases the dual connectivity, the mobile station 300 refers to thereception state information and transmits the user data that has not yetbeen received by the macro base station 100 to the macro base station100 with the proper quantity. Consequently, even when switching from thedual connectivity to the single connectivity occurs, it is possible toprevent duplication and loss of user data that is to be received by themacro base station 100.

As described above, according to the embodiment, if an error is detectedin the transmission path via the small base station at the time of dualconnectivity, the macro base station transmits, to the mobile station,the reception state information that specifies unreceived packets andreceived packets. Then, if the macro base station releases the dualconnectivity and switches to the single connectivity, mobile stationrefers to the reception state information and transmits the packet thathas not been received by the macro base station. Consequently, even ifswitching of connection occurs, it is possible to transmit packets withthe proper quantity from the mobile station to the macro base stationand it is possible to prevent duplication and loss of user data.

Furthermore, in the third embodiment described above, an error isdetected by the macro base station 100; however, an error may also bedetected by, for example, the small base station 200 or the mobilestation 300. Furthermore, the small base station 200 or the mobilestation 300 that has detected the error may also notify the macro basestation 100 of the occurrence of the error. The small base station 200detects an error in a case in which, for example, the number ofretransmissions with the mobile station 300 in the RLC layer reaches thepredetermined maximum number of retransmissions, or the like.Furthermore, the mobile station 300 detects an error in a case in which,even after the predetermined time has elapsed since the user data wastransmitted to the small base station 200, the reception confirmation(ACK) of the user data is not received from the small base station 200,or the like.

Furthermore, in each of the embodiments, an example of the dualconnectivity in which the mobile station 300 simultaneously connects twobase stations, i.e., the macro base station 100 and the small basestation 200, has been described; however, the same process can also beperformed in multiple connections in which the mobile station 300simultaneously connects to three or more base stations. Namely, if anerror is detected in a transmission path via one of the base stations,the mobile station on the reception side transmits reception stateinformation to the primary base station to which the control plane isconnected. Then, the primary base station releases the multipleconnections, switches to the single connectivity, and refers to thereception state information, whereby the primary base station cantransmit the user data with the proper quantity to the mobile station.

Furthermore, in each of the embodiments, if an error is detected in thetransmission path via the small base station 200, the macro base station100 decides to release the dual connectivity and switch to the singleconnectivity. However, if an error is detected, the macro base station100 does not always need to switch to the single connectivity. In a caseof not switching to the single connectivity, the mobile station 300 orthe macro base station 100 on the reception side transmits the receptionstate information to the macro base station 100 or the mobile station300 on the transmission side, whereby the transmission side can confirmthe reception state of the packets on the reception side.

Furthermore, in each of the embodiments, it is assumed that, if an erroris detected in the transmission path via the small base station 200, thedual connectivity is released and the connection is switched to thesingle connectivity; however, the occurrence of a connection switch isnot limited to a case in which an error is detected. Namely, forexample, it is conceivable that, in also a case in which the mobilestation 300 is moved and thus the macro base station or the small basestation at the connection destination is changed, the dual connectivityis temporarily released and the connection is switched to the singleconnectivity. Furthermore, in such a case, also, the reception sidetransmits the reception state information to the transmission side,whereby it is possible to prevent duplication and loss of user data atthe time of the occurrence of a connection switch.

The physical configuration of the macro base station 100, the small basestation 200, and the mobile station 300 in each of the embodiments doesnot always need to be the same block diagram as that illustrated in FIG.2 and FIG. 3. Thus, a specific example of the hardware configuration ofthe macro base station 100, the small base station 200, and the mobilestation 300 will be described.

FIG. 9 is a block diagram illustrating the hardware configuration of abase station. The base station illustrated in FIG. 9 corresponds to, forexample, the macro base station 100 and the small base station 200 andincludes an antenna 501, a control unit 502, a radio frequency (RF)circuit 503, a memory 504, a CPU 505, and a network interface 506.

The control unit 502 implements the function performed by, for example,the control unit 14 in the macro base station 100 and the control unit24 in the small base station 200.

The network interface 506 is an interface for connecting to another basestation by the wired connection. For example, the macro base station 100and the small base station 200 are connected in a wired manner via thenetwork interface 506.

The CPU 505, the memory 504, and the RF circuit 503 implement thefunction performed by, for example, the communication unit 11 in themacro base station 100 and the communication unit 21 in the small basestation 200. Namely, for example, in the memory 504, various kinds ofprograms, such as the programs for implementing the function performedby the communication unit 11 or the communication unit 21, are stored.Then, the CPU 505 reads the programs stored in the memory 504 andcooperates with the RF circuit 503 or the like, whereby implementing thefunction performed by the communication unit 11 or the communicationunit 21.

FIG. 10 is a block diagram illustrating the hardware configuration of amobile station. The mobile station illustrated in FIG. 10 correspondsto, for example, the mobile station 300 and includes an antenna511, acontrol unit512, an RF circuit 513, a memory 514, and a CPU 515.

The control unit 512 implements the function performed by, for example,the control unit 34 in the mobile station 300.

The CPU 515, the memory 514, and the RF circuit 513 implements thefunction performed by, for example, the communication unit 31 in themobile station 300. Namely, for example, in the memory 514, variouskinds of programs, such as the programs for implementing the functionperformed by the communication unit 31. Then, the CPU 515 reads theprograms stored in the memory 514 and cooperates with the RF circuit 513or the like, whereby implementing the function performed by thecommunication unit 31.

According to an aspect of an embodiment of the radio communicationdevice and the radio communication method disclosed in the presentinvention, an advantage is provided in that it is possible to preventinconsistency of user data that is transmitted and received.

All examples and conditional language recited herein are intended forpedagogical purposes of aiding the reader in understanding the inventionand the concepts contributed by the inventor to further the art, and arenot to be construed as limitations to such specifically recited examplesand conditions, nor does the organization of such examples in thespecification relate to a showing of the superiority and inferiority ofthe invention. Although the embodiments of the present invention havebeen described in detail, it should be understood that the variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A radio communication device comprising: acommunicator configured to have a wireless connection with a firstcommunication device and a second communication device to perform radiocommunication; and a controller configured to perform a control forconnection change in the radio communication, when the communicatorreceives information on a change in the wireless connection from thefirst communication device, the controller performing the control forexecuting a process corresponding to reception state informationreceived by the communicator in accordance with the connection change,the reception state information specifying data that has already beenreceived or data that has not yet been received among data that has beentransmitted to the first communication device or the secondcommunication device.
 2. The radio communication device according toclaim 1, wherein the communicator receives the reception stateinformation after the connection change.
 3. The radio communicationdevice according to claim 1, wherein the communicator receives an RRCsignal including information on the change in the wireless connection.4. The radio communication device according to claim 1, wherein thecommunicator receives an RRC signal including information on theconnection change and the reception state information.
 5. The radiocommunication device according to claim 1, wherein the communicatortransmits data to which an identifier indicating an order of the data isadded, and receives the reception state information that includestherein both the identifier of the first unreceived data and informationthat indicates whether a predetermined number of pieces of data to eachof which an identifier indicating that the order is subsequent to theidentifier of the first unreceived data has already been received. 6.The radio communication device according to claim 1, wherein thecommunicator receives first data via a first path between the radiocommunication device and the first communication device, and receivessecond data from the first communication device via the secondcommunication device.
 7. The radio communication device according toclaim 1, wherein the reception state information is related to data lostbetween the first communication device and the second communicationdevice, and is transmitted from the first communication device to thecommunicator.
 8. A radio communication device comprising: a communicatorconfigured to connect to a first communication device and a secondcommunication device and to perform radio communication with the firstcommunication device and a controller configured to perform a controlrelating to a wireless connection with the first communication device,the communicator transmitting, to the first communication device,information on connection change with the first, communication device,and transmitting, to the first communication device in accordance withthe connection change, reception state information specifying data thathas already been received or data that has not yet been received amongdata that has been transmitted by the first communication device or thesecond communication device.
 9. The radio communication device accordingto claim 8, wherein the communicator is configured to receive data fromthe first communication device via the second communication device. 10.The radio communication device according to claim 8, wherein thecommunicator receives an RRC signal including information on the changein the wireless connection.
 11. The radio communication device accordingto claim 8, wherein the communicator transmits an RRC signal includinginformation on the connection change and the reception stateinformation.
 12. The radio communication device according to claim 8,wherein the communicator receives data to which an identifier indicatingan order of the data is added, and transmits the reception stateinformation that includes therein both the identifier of the firstunreceived data and information that indicates whether a predeterminednumber of pieces of data to each of which an identifier indicating thatthe order is subsequent to the identifier of the first unreceived datahas already been received.
 13. The radio communication device accordingto claim 8, wherein the reception state information is related to datalost between the first communication device and the second communicationdevice.
 14. A radio communication system comprising a firstcommunication device, a second communication device and a thirdcommunication device, wherein the first communication device comprises:a first communicator configured to have a wireless connection with thesecond communication device and the third communication device toperform radio communication; and a first controller configured toperform a control for connection change in the radio communication, whenthe first communicator receives information on a change in the wirelessconnection from the second communication device. the first controllerperforming the control for executing a process corresponding toreception state information received by the first communicator inaccordance with the connection change, the reception state informationspecifying data that has already been received or data that has not yetbeen received among data that has been transmitted to the secondcommunication device or the third communication device, and the secondcommunication device comprises: a second communicator configured toconnect to the first communication device and the third communicationdevice and to perform radio communication with the first communicationdevice; and a second controller configured to perform a control relatingto a wireless connection with the first communication device, the secondcommunicator transmitting, to the first communication device,information on connection change with the first communication device,and transmitting, to the first communication device in accordance withthe connection change, reception state information specifying data thathas already been received or data that has not yet been received amongdata that has been transmitted by the first communication device or thethird communication device.
 15. The radio communication system accordingto claim 14, wherein the reception state information is related to datalo between the first communication device and the second communicationdevice.